Java finally块

try块也可以有零个或一个finally块。 finally块总是与try块一起使用。

语法

finally块的语法是：

finally  {
    // Code for finally block 
}

finally块以关键字finally开始，后面紧跟一个大括号和一个大括号。finally块的代码放在大括号内。

try，catch和finally块有两种可能的组合：try-catch-finally或try-finally。try块可以后跟零个或多个catch块。try块最多可以有一个finally块。try块必须有一个catch块和一个finally块或两者都有。try-catch-finally块的语法是：

try  {
    // Code for try block
}
catch(Exception1 e1)  {
    // Code for catch block
}
finally  {
    // Code for finally block
}

try-finally块的语法是：

try  {
    // Code for try block
}
finally  {
    // Code for finally block
}

无论在相关联的try块和/或catch块中发生什么，finally块中的代码都保证执行。所以，一般使用finally块来写清理代码。
例如，可能用来释放一些资源，如数据库连接的关闭，文件的关闭等，当完成它们时，必须释放。

在try-finally块允许实现这个逻辑。代码结构将如下所示：

try  {
    // Obtain   and  use  some resources here
}
finally  {
    // Release the   resources that were  obtained in the   try  block
}

示例-1

下面的代码演示了finally块的使用。

public class Main {
  public static void main(String[] args) {
    int x = 10, y = 0, z = 0;
    try {
      System.out.println("Before dividing x  by  y.");
      z = x / y;
      System.out.println("After dividing x  by  y.");
    } catch (ArithmeticException e) {
      System.out.println("Inside  catch block a.");
    } finally {
      System.out.println("Inside finally  block a.");
    }

    try {
      System.out.println("Before setting  z  to 2.");
      z = 2;
      System.out.println("After setting  z  to 2.");
    }
    catch (Exception e) {
      System.out.println("Inside  catch block b.");
    } finally {
      System.out.println("Inside finally  block b.");
    }
    try {
      System.out.println("Inside try block c.");
    }
    finally {
      System.out.println("Inside finally  block c.");
    }

    try {
      System.out.println("Before  executing System.exit().");
      System.exit(0);
      System.out.println("After  executing System.exit().");

    } finally {
      // This finally block will not be executed
      // because application exits in try block
      System.out.println("Inside finally  block d.");
    }
  }
}

上面的代码生成以下结果。

Before dividing x  by  y.
Inside  catch block a.
Inside finally  block a.
Before setting  z  to 2.
After setting  z  to 2.
Inside finally  block b.
Inside try block c.
Inside finally  block c.
Before  executing System.exit().

重新抛出一个异常

捕获的异常可以被重新抛出。

public class Main {
  public static void main(String[] args) {
    try {
      m1();
    } catch (MyException e) {
      // Print the stack trace 
      e.printStackTrace();
    }
  }

  public static void m1() throws MyException {
    try {
      m2();
    } catch (MyException e) {
      e.fillInStackTrace();
      throw e;
    }
  }

  public static void m2() throws MyException {
    throw new MyException("An  error has  occurred.");
  }
}

class MyException extends Exception {
  public MyException() {
    super();
  }

  public MyException(String message) {
    super(message);
  }

  public MyException(String message, Throwable cause) {
    super(message, cause);
  }

  public MyException(Throwable cause) {
    super(cause);
  }
}

上面的代码生成以下结果。

MyException: An  error has  occurred.
    at Main.m1(Main.java:14)
    at Main.main(Main.java:4)